The present invention relates to a device structure of surface acoustic wave devices used as filters of mobile communication devices, etc.
Filters in the form of surface acoustic wave devices are widely used in communication apparatus, broadcasting apparatus, measuring apparatus, etc. because of their small-size, lightness, and high performance. Conventionally severe narrow-band characteristics, and additionally small temperature changes at center frequencies and severe attenuation characteristics outside the pass band are required of intermediate frequency filters used in analog communication systems. In such applications transversal surface acoustic wave filters, etc. having 0.03-0.1% of a 3 dB specific band width (hereinafter called specific band width) on ST cut crystal substrates are used.
In mobile communication, etc., replacement of analog communication systems by digital communication system is being studied for more efficient uses of frequencies and privacy communications. Relatively wide specific band width as wide as 0.3-0.5%, flat group delay times, and attenuation characteristics outside the pass band whose values exceed those of the analog communication system are required of intermediate frequency filters used in a digital communication system. Portable apparatus for mobile communication must be smaller-sized, and the surface acoustic wave filters must be accordingly smaller-sized and have smaller insertion losses.
The conventional transversal surface acoustic wave filter including two interdigital transducers has an advantage that a required amplitude characteristic and a phase characteristic can be designed independently from each other. But a disadvantage of the transversal surface acoustic filter is large insertion losses, and to obtain a 0.3-0.5% band width, a number of finger electrodes are necessary, which makes the device large-sized and its attenuation outside the pass band insufficient.
A known structure of filters with low losses and wide bands includes three interdigital transducers each having four pairs of finger electrodes neighboring on a LiNbO.sub.3 substrate. Its insertion loss is 10 dB, and a specific band width is 20% (M. F. Lewis, Electron Letters, vol. 8, no. 23, p. 553 (1972). But too large insertion loss and specific band width are disadvantages of this filter.
On the other hand, known small-sized surface acoustic wave filters with small losses include resonator filters and multi-electrodes interdigital transducer filters. These filters have structures which have been merely determined in consideration of insertion losses and attenuation outside the pass band, and have failed to have required group delay time characteristics. For example, a filter having three interdigital transducers each having 700 pairs of finger electrodes has a 5 dB insertion loss, and a 0.02% specific band width (Yamada, Yoshikawa, Ishihara, Report of Institute of Japan Electronics and Communication Engineers, vol. J60-A, no. 9, pp. 805 (1977)). But its group delay time characteristic and amplitude ripple in pass band are unknown. Its specific band width is narrow.
As described above, the surface acoustic wave filter of the conventional structure can not attain relatively wide specific band widths, large attenuation outside the pass band, small-sizes and low insertion losses, and additionally flat group delay time characteristics.